Vitamin D deficiency has been linked to the development of cardiovascular disease and it is estimated that six million children are currently vitamin D deficient. As it is difficult for many children to consume the new recommended intake of 400 IU/d set forth by the American Academy of Pediatrics, many have suggested that vitamin D supplementation is necessary. Vitamin D deficiency is even more common in northern latitudes, amongst some minority groups with darker skin pigmentation, and in those who are overweight or obese. In these groups, higher supplemental doses may be needed to maintain optimal serum levels and to prevent cardiovascular risk. The overall objective of the proposed work is to determine the appropriate vitamin D supplementation requirements for children living at northern latitudes (above 42Z) to maintain optimal serum levels of 25-hydroxyvitamin D [25(OH)D] and minimize cardiometabolic risk. We will also investigate for the first time in a randomized controlled trial what happens to serum 25(OH)D and cardiometabolic risk factors after supplementation is discontinued. Fourth-eight grade children from socioeconomically disadvantaged and racially diverse communities with high obesity rates in the northeastern U.S. will be randomized to 400 IU, 1000 IU, or 2000 IU/d for six months starting in the early winter. Serum 25(OH)D and cardiometabolic risk factors (blood lipids and glucose) will be measured at baseline prior to supplementation, at 3- and 6-months, and then again 6 months following the discontinuation of supplementation. Adiposity, physical activity, dietary intake of vitamin D, and sun exposure will also be assessed at these times. Results of this work will inform the development of evidence-based recommendations and guidelines regarding vitamin D supplementation to maintain optimal 25(OH)D levels and to reduce cardiometabolic risk. If vitamin D repletion has the potential to improve the cardiometabolic risk profile during childhood, then we will ultimately be able to lower a child's risk of developing cardiovascular disease in adulthood. PUBLIC HEALTH RELEVANCE: Vitamin D deficiency has been linked to the development of cardiovascular disease and it is estimated that six million children are currently vitamin D deficient. This research will help to determine what supplemental doses of vitamin D may be necessary to raise serum levels to optimal levels and how vitamin D supplementation and changes in vitamin D status impact cardiovascular risk factors in schoolchildren.